This invention relates to a radiation detecting apparatus and an apparatus of radiation tomography which uses the same. The following description is with reference to an example of a medical X-ray CT (computerized tomography) for simplicity of explanation.
Recently, there has been proposed, as an X-ray detector for an X-ray CT apparatus, a solid state detector formed of a scintillator and a photodiode. This detector is drawing attention because of its inherently satisfactory signal-to-noise ratio in dealing with the image. In another X-ray CT apparatus based on third generation technology, the variability of sensitivity of X-ray detectors among channels results in differences of the concentration of a image, causing the emergence of ring artifact which is a noise attributable to the concentration difference appearing as concentric circles due to the rotation of the detector. A method of signal modification for alleviating this impropriety is proposed in Japanese Patent Unexamined Publication No. 53-68091, for example.
However, as clinical apparatus are gaining in performance and function these days, X-ray CT apparatus is required to have its imaging operation sped up. Consequently, the X-ray exposure scheme is altered from pulse X-ray exposure to continuous X-ray exposure, and conventional signal calibration alone cannot reduce the ring artifact enough to meet the sophisticated clinical maneuvers. This problem will be explained with reference to FIGS. 1A and 1B.
FIG. 1A is a diagram showing, in the sense of a model, a pulse X-ray exposure scheme. Initially, the sensitivity of the detector is measured in the absence of a body under test. This measurement is for the acquisition, for each sensor, of offset data O.sub.1 in the absence of X-ray exposure and reference data R in the presence of X-ray exposure but in the absence of a body under test. In the case of an array of detectors of m in number, m pieces of offset data O.sub.1 and m pieces of reference data R are collected. The difference of each set of data (R-O.sub.1) is evaluated as reference calibration (or air calibration) data. Also in the measurement of the magnitude of X-rays transmitted through a body under test, offset data without X-ray exposure (O.sub.21, O.sub.22, O.sub.23, . . . ) and data with X-ray exposure (S.sub.1, S.sub.2, . . . ) of the detectors are collected to evaluate difference values (S.sub.i -O.sub.2i) as signal components. Projection data, which is a set of signals after offset calibration and sensitivity calibration, is obtained as ##EQU1## by calculating log ##EQU2## from the air calibration data and signal components after logarithm conversion i.e., log(S.sub.i -O.sub.2i) and log(R-O.sub.1).
However, as mentioned previously, as X-ray CT apparatus have gained in imaging speed recently, pulse X-ray exposure is being replaced with continuous X-ray exposure as shown in FIG. 1B. In consequence, the acquisition of offset data for each channel is no more longer carried out, and projection data is now obtained as ##EQU3## by using a representative offset data O.sub.2 which is collected immediately before tomography.
A problem emerging here is that the variation in the sensitivity of X-ray detectors due to temperature changes during the continuous X-ray exposure imposes an adverse influence on the accuracy of signal detection. This is primarily attributable to a large variation in the sensitivity of the X-ray detector caused by a temperature change. A technique pertinent to the present invention for maintaining the detector temperature constant is disclosed in JP-A-62-3684. This technique is intended to heat the detector to a certain temperature before tomography is conducted, and heating is suspended during the actual tomographic operation so as to avoid inductive noises generated by the heater. When this technique is applied to a pulse X-ray exposure scheme, in which the offset is measured at each X-ray exposure for use as the offset calibration data, no ring artifact occurs even if the detector temperature changes during tomography. However, when this technique is applied to a continuous X-ray exposure scheme, in which the offset is measured only once before tomography, the offset data has a degraded accuracy if the detector temperature changes during X-ray exposure, creating a ring artifact.